I don't recall any games making use of Ati's R600 tessellation features, though (at least outside of XBox360, which is D3D-only). I recall running Neverwinter Nights and Morrowind could make use of R200/R300-level tessellation, but that feature was removed on Catalyst 5.1 or so.OpenGL users have been using Ati's tesselation features while Microsoft was still promoting D3D10 as the big revolution. Eventually it was standardized and beat into shape for D3D11 and OpenGL 4.0. Yes, standardization made it better. That's awesome.
Do prove me wrong if you have examples, but right now I feel it's kind of a moot point - noone was using hardware tessellation on the PC prior to D3D11.